Many laser applications, particularly those in the field of laser spectroscopy and coherent laser combining, require stable single transverse and longitudinal mode operation of a laser, which is often referred to as “single-frequency operation”. In some applications, the need for single transverse mode operation is a consequence of strict requirements on beam shape, while single longitudinal mode operation is needed to avoid mode beating and provide stable continuous wave operation. Additionally, the transverse modes of a beam can have different resonance frequencies than the fundamental mode, or be phase-shifted therefrom due to Gouy phase shift, which can also result in mode beating.
Cavity Ring-Down Spectroscopy (CRDS), a powerful method for measuring concentrations of trace constituents in a gas mixture, for example the trace chemicals present in human breath, is an example of an application requiring single-frequency operation. The CRDS technique requires a laser source capable of emitting a pulsed or continuous wave and sequentially producing numerous lines with narrow linewidth, and a laser cavity/resonator with highly reflective mirrors to enable decay time measurements of each laser line inside the cavity. The gas mixture to be analyzed is then introduced into the cavity and the decay time of the laser lines can be measured and compared against the known decay time for an empty cavity. The analysis of decay times enables the identification and quantification of constituents in the gas mixture.
The CRDS technique works best with a tunable single-frequency laser source with a narrow linewidth. One method of resonance line selection is utilizing a rotatable intra-cavity diffraction grating that selects one spectral region that is fed back into the resonator, which in turn outputs a laser beam of the selected frequency spectrum. The output beam may be a single lowest-order mode emission, or some mode mixing may take place. Frequently, the extra modes are transverse modes which are excited and have not been excluded due to the line selection method, such as the diffraction grating, not being sufficiently fine-tuned for excluding them.
Existing methods for obtaining single-frequency operation such as Master Oscillator Power Amplifiers (MOPA) or injection-locked lasers are complex, expensive, and difficult to use. Methods requiring intra-cavity elements to filter out higher-order modes, such as using a hard aperture, result in loss of power of the resultant beam and are not suitable for a broad wavelength range. Single-mode fibers provide another means for achieving single-mode operation. However, their availability is severely limited for some wavelengths, for example, in the mid infra-red range.
Methods such as that disclosed in U.S. Pat. No. 6,324,191 (the '191 Patent), aimed for ophthalmic surgery, teach a means to achieve single transverse mode operation of a passively mode-locked Nd3+-doped glass laser. In the '191 Patent, single-frequency operation is obtained by detecting a beat-note in the emitted laser beam to determine the co-existence of modes therein, and advancing a straight edge towards the center of the beam until beat-note frequencies are no longer detected, thus indicating the successful suppression of higher-order transverse modes. The mode suppression method taught in the '191 Patent is based on preferential losses between transverse modes, and therefore makes the output power smaller, which is an undesirable effect. Further, the said method is intrinsically multi-frequency, and does not provide true single-frequency operation, as the use of a straight edge to suppress transverse modes cannot completely isolate the fundamental mode, due to the close spacing of the fundamental and transverse modes.
There is a need for a method and apparatus that provides true single-frequency operation in an economical, easy-to-use manner, and with a minimal loss of output power.